A basic microwave circuit is the hybrid Tee comprising four terminals or arms so connected that when power is applied to any one arm, it divides approximately equally between the two arms adjacent to it but is completely isolated from the arm opposite from it. This type of circuit is commonly constructed in waveguide and/or coaxial line. A coaxial version is shown in U.S. Pat. No. 2,935,702. Another analogous device is the rat-race which has three of the four transmission lines between output ports equal to one-quarter wavelength and one line equal to three quarter wavelengths at midband. The isolation between any two opposite ports is infinite at midband because the two path lengths differ by exactly 180.degree.. The isolation is degraded severely with a change in frequency, however, due to the change in relative path lengths. The useful bandwidth of such devices is about 2%.
A microstrip version of the ring coupler has been disclosed by John Reindel in U.S. Pat. No. 4,023,123. That device utilizes a 180.degree. twisted pair of transmission lines and has a wider operational bandwidth than other prior art devices and is useful at EHF. The aforementioned reverse phased ring coupler, however, has the disadvantage that the manufacturing of the twisted pair of parallel conductors requires a skilled craftsman to position, space, and orient the twisted conductor pair under a microscope for each reverse phase ring produced.